Diaphragms used in speakers differ in shape depending on their use. Examples of the diaphragm are a cone diaphragm, a flat-plate diaphragm, and a dome diaphragm. A dome speaker using the dome diaphragm is characterized in that a diameter thereof can be reduced since a driving point is located on an outer periphery thereof. Since the dome diaphragm has a dome shape, it is difficult to increase the strength thereof by a multi-layer structure including a honeycomb layer as in the flat-plate diaphragm, and a method for increasing the strength of the dome diaphragm is limited. The simplest method for increasing the strength of the dome diaphragm is to increase the thickness of the diaphragm. However, if the dome diaphragm increases in thickness, this causes some downsides, i.e., it is difficult for such dome diaphragm to reproduce high-frequency sounds, and the cost increases. Here, Patent Document 1 proposes a diaphragm in which a dome portion is provided with a reinforcing rib. In accordance with this configuration, because the dome portion is reinforced by the rib, the dome diaphragm other than the rib can be reduced in thickness.    Patent Document 1: Japanese Utility Model Application Publication No. 53-82329